OBJECTIVE To evaluate a human growth hormone variant immunoassay for use with pregnant rhesus monkeys. RESULTS The human and nonhuman primate placenta expresses a unique member of the growth hormone gene cluster, the placental growth hormone-variant (GH-V). The physiology of this molecule remains incompletely understood in human or primate pregnancy. We wish to develop the rhesus monkey as an experimental model to investigate the role of this hormone in maternal-fetal communication and regulation of growth and metabolism. An immunoradiometric assay (IRMA) for human placental growth hormone was obtained from Biocode Biotechnology, Liege, Belgium. We determined that nonpregnant monkey serum does not contain cross-reacting materials, whereas late pregnancy serum contained high amounts of immunoactivity. Weekly blood samples obtained from an individual rhesus monkey showed that serum levels were not detectable until midpregnancy, after which they rose dramatically to a peak in mid-third "trimester", when an experimental cesarean section was conducted. FUTURE DIRECTIONS Additional studies are needed to confirm parallelism of the human standard with rhesus serum, to confirm the results with this monkey with additional subjects, and to use cloned rhesus placental hormone cDNAs to identify specificity for rhesus growth hormone family members. KEY WORDS placenta, placental growth hormone, hormone secretion, pregnancy, fetal growth FUNDING NIH R01 HD26458